1. Field of the Invention
The present invention is generally related to an ionizer for removing electric charges, and more specifically to an ionizer that has been improved to allow reduction of ripples and of balance shift without degrading the capability as an ionizer.
2. Description of the Background Art
FIG. 5 is a concept diagram of a conventional ionizer. The ionizer has a main body 1. Main body 1 is provided with a plurality of acicular electrodes 2. Main body 1 normally is provided with a grounded metallic cover (not shown). The ionizer is used for the purpose of removing electric charges. During the manufacturing process of a semiconductor device, static electricity is generated on a substrate 3. If the static electricity is not removed, the device could be destroyed, or particulate dust may adhere to the semiconductor device so that the yield is reduced, which leads to lower productivity. In order to solve this problem, an ionizer blows an ionic substance against substrate 3, the target of electric charge removal, to remove the electric charge.
Most of the conventional ionizers is of the type that generates ions by applying a high direct current or alternating current voltage to an acicular electrode 2 or to a thin metal line electrode and generating corona discharge in the vicinity of electrode 2 to ionize the atmosphere. Most ionizers create corona discharge constantly regardless of the surrounding environment. Thus, a grounded electrode having an unchanging area of contact with the ions is provided in the vicinity of discharge electrode 2. Such an electrode is referred to as a ground electrode, and is conventionally and in many cases formed integrally with the metallic cover.
Broadly, the use of a conventional ionizer for removal of electric charges involves the two following problems.
One problem is as follows. An ionizer 10 as shown in FIG. 5 normally generates positive ions and negative ions, and when a high alternating current voltage as shown in FIG. 6A is applied to discharge electrode 2, the surface potential of substrate 3 varies according to the application of the high voltage to discharge electrode 2 due to the ions that arrive at substrate 3 which is the target object of electric charge removal and to the induction by discharge electrode 2 that has attained the high voltage level.
Alternate application of positive ions and negative ions causes the surface potential of substrate 3 to vary in the form of waves as shown in FIG. 6B. This wave-like surface potential displacement will be hereinafter referred to as ripples.
In particular, in the case where ionizer 10 having high capability is used, the ripples are small if the electrostatic capacity of the target object of electric charge removal (i.e. the substrate) is large. If the electrostatic capacity of the target object of electric charge removal is small, however, the ripples would create a surface potential variation of xc2x1several 100 volts. This can be easily seen from the relation of Q=CV (Q: electric charge, C: electrostatic capacity, V: potential difference).
Even when the electrostatic capacity of the target object of electric charge removal (substrate 3) is the same, the ripples are small in the case in which substrate 3 makes contact with a ground as shown in FIG. 7A since electrostatic capacity is large. On the other hand, when substrate 3 or the target object of electric charge removal is raised from the ground as shown in FIG. 7B, the electrostatic capacity becomes small and the ripples extremely large. When the ripples become large, i. e. when surface potential displacement becomes large, such that a discharge or the like occurs, the device is destroyed, which results in reduction of the yield.
In this manner, when the charged amount of the target object of electric charge removal is small, and when the amount of ripple variation is great, the surface potential can increase more than the initial charged amount.
Another problem is that, when an ionizer is used continuously, the balance of discharge of the positive ions and the negative ions is destroyed (a condition hereinafter referred to as xe2x80x9cbalance shiftxe2x80x9d) due to the degradation of discharge electrodes 2 or the adhesion of foreign substance to discharge electrodes 2. The continued use of the ionizer with its balance of discharge destroyed would result in reversely charging the target object of electric charge removal positively or negatively, as shown in FIG. 8. In other words, while the dotted line should be at 0 volt, the ripple curve may undergo parallel translation upward, i.e. the target object of electric charge removal may be reversely charged, for instance by 100 V, toward the positive side. In particular, in the case of an ionizer having high ion generation capability, large surface potential will be generated when the electrostatic capacity of the target object of electric charge removal that has been reversely charged due to this loss of balance becomes small. When large surface potential is created on the target object of electric charge removal, thereby causing a discharge and the like, the device could be destroyed, or particles may adhere to the semiconductor device, which leads to the problem of reduced yield.
The present invention is made to solve the above-described problems, and its object is to provide an ionizer that has been improved to allow reduction of ripples and of balance shift without degrading the capability as an ionizer.
According to the first aspect of the present invention, an ionizer has a main body. A discharge electrode for generating ions by creating corona discharge with high voltage application is mounted on the main body. A grounded ground electrode that makes contact with the ions for absorbing a portion of the ions is provided in the vicinity of the discharge electrode. The ionizer is provided with an element for changing the area of contact between the ground electrode and the ions.
Since the element for changing the area of contact between the ground electrode and the ions is provided according to the present invention, the area of the ground electrode can be changed so as to effect adjustment to reduce ripples and balance shift while limiting the degradation of the capability of an ionizer as much as possible.
According to the second aspect of the present invention, the ionizer is further provided with an element for changing the distance between the ground electrode and the discharge electrode.
Since the element for changing the distance between the ground electrode and the discharge electrode is provided according to the present invention, the amount of a portion of ions to be absorbed by the ground electrode can be adjusted.
According to the third aspect of the present invention, the ionizer is further provided with an element for moving and fixing the ground electrode.
In the ionizer according to the fourth aspect of the present invention, the element for moving and fixing the ground electrode includes a bolt erected on the main body and an elongate hole being provided in the ground electrode and extending in the up-down direction or in the right-to-left direction. The bolt engages the elongate hole, thereby fixing the ground electrode to the main body such that the ground electrode is movable in the up-down direction or in the right-to-left direction.
Since the ground electrode is fixed such that it is movable in the up-down direction or in the right-to-left direction, the area of contact between the ground electrode and the ions can be changed.
In the ionizer according to the fifth aspect of the present invention, between the ground electrode and the main body, a spacer is provided for changing the distance between the ground electrode and the discharge electrode.
According to the present invention, the provision of a spacer allows changing of the distance between the ground electrode and the discharge electrode.
In the ionizer according to the sixth aspect of the present invention, a cover for covering the main body is provided to the main body. The ground electrode is fixed to the main body with the cover existing therebetween.
According to the present invention, a conventional ionizer itself can be utilized with the ground electrode fixed to the main body of the conventional ionizer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.